Hitherto, a magnetic toner to be used for forming an image by a magnetic one-component jumping development method has been required to have high fluidity so as to achieve stable supply to a developing sleeve, image density, and image stabilization, and as an external additive for imparting the fluidity, an external additive having a small particle diameter has been frequently used. However, the external additive having a small particle diameter involves a problem in that when the magnetic toner is transferred onto a medium, a great amount of a transfer residual toner remains on a drum (electrophotographic photosensitive member), and hence the consumption amount of the magnetic toner increases in order to satisfy image density, with the result that printing cost per sheet becomes high.
Further, in recent years, there has been a demand for higher speed and longer life in copying machines, printers, and the like, and it is predicted that shear which is more than before is applied to the magnetic toner between the developing sleeve and a toner regulating blade. Therefore, when the external additive having a small particle diameter is used in the same way as before, it is predicted that the external additive having a small particle diameter adhering to the surface of the magnetic toner is buried, and the external additive does not serve as an external additive. As a result, the transfer property is deteriorated during long-term use, which may cause image quality defects, with the result that there is a concern that satisfactory image density may not be obtained, and the consumption amount of the magnetic toner may increase further compared to that of the initial stage.
In order to solve the above-mentioned problem, in recent years, there has been proposed a monodispersed spherical external additive having a large particle diameter replacing the external additive having a small particle diameter (for example, PTL 1). However, when a toner using the monodispersed spherical external additive is applied to the magnetic one-component jumping development method, although the toner consumption amount is suppressed by the improvement of initial transfer efficiency, there is a possibility that the transfer property is deteriorated during long-term use, which may cause image quality defects.
In order to solve the above-mentioned problem, there have been proposed various procedures such as a procedure for enhancing adhesion strength during an external addition step and a procedure for changing the shape of an external additive itself.
For example, PTL 2 discloses a method of fixing inorganic fine powder having a large particle diameter to the surface of a toner particle by applying strong shear in a gap between a rotation drive part in an external additive mixing tank and a casing. However, this procedure is not necessarily effective for a pulverized toner, and the inorganic fine powder is rolled to a recess of the toner particle due to the strong shear force in the gap between the rotation drive part and the casing, with the result that there is a possibility that the inorganic fine powder may not serve as an external additive sufficiently.
For example, PTL 3 provides an example in which non-spherical amorphous silica having a large particle diameter is externally added so as to suppress the above-mentioned burial and rolling. However, when this example is applied to the magnetic one-component jumping development method, sliding between the developing sleeve and the toner regulating blade is stronger than that between two-component developers, and the external additive may be separated or packing between toners may occur. As a result, the developing property and transfer property are degraded, and there is a possibility that problems such as a white streak and density unevenness may occur.
In addition, PTL 4 and PTL 5 disclose examples using an organic-inorganic composite particle, in which an inorganic particle adheres to the surface of an organic particle, as a spacer particle. However, considering the future high speed and long life, when the composite particle is externally added to a negatively chargeable magnetic toner particle, the chargeability under a high-temperature and high-humidity environment may be degraded when the composite particle is a positively chargeable particle (PTL 4). Further, even when the composite particle is a negatively chargeable particle, there still remains room for improvement in the case of assuming further increases in speed and life (PTL 5).
Considering the foregoing, there still remains room for improvement so as to satisfy both the initial transfer property and durable stability and the stability of image quality in the magnetic one-component jumping development method.